for planning purposes use 5-10% of the drainage area. larger sites/less impervious coverage require smaller percentages, smaller sites/more impervious coverage require larger percentage...

the Rational Method (for sites/watersheds up to several hundred acres) is:

q=CiA
will provide the runoff volume in cubic feet per second, which can be converted to total runoff by multiplying by storm length in seconds

C=runoff coefficient
i=rain intensity in inches/hour of design storm
A=site area in acres
1 acre = 43,560 square feet

Other factors in sizing a pond is the duration of storage - typically 72 hours is used, amount of acceptable bounce, and topography.

the same 5-10% will cover other BMPs including rain gardens, treatment wetlands, infiltration basins (depends on soil), et cetera...

Thanks Barry. Exactly what I needed. Thanks.

I plugged in my numbers to this site for approx. 10 acres of runoff area on a 27 acre site. The calculator put me at about 2.7 acres of pond which is at your 10% mark for planning. Holy shit! Didn't imagine that almost 3 acres would have to be dedicated to a retention pond. That seems huge.

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